Chapter 18 Lecture Notes - CEM 252 at MSU Spring 1997

Chapter 18-Slide 1

Carbohydrates - from simple to complex

• Monosaccharides: single-unit carbohydrates (monomers)
  • aldoses: carbohydrates with aldehyde group
  • ketoses: carbohydrates with ketone functionality
• Polysaccharides: chains of monosaccharides (polymers)
  • examples: starch, cellulose, glycogen

Chapter 18-Slide 2

Carbohydrates - Stereochemistry

D/L system used for penultimate carbon

1. put most oxidized carbon at top
2. continue carbon chain to bottom
3. non-hydrogen substituent on right=D

Trivial names indicate all other stereocenters

Chapter 18-Slide 3

Monosaccharides of Different Sizes

Chapter 18-Slide 4

Common Monosaccharides

Chapter 18-Slide 5

Amino Sugars

Chapter 18-Slide 6

Chapter 17 Review: Acetal and Hemiacetal Formation

• This equilibrium is acid-catalyzed (both directions)
  Oxygen is protonated, making the carbon more reactive
• The most abundant species at equilibrium is usually the ketone or aldehyde
  exception: cyclic acetals

Chapter 18-Slide 7

Challenge

Draw a hemiacetal form of glucose - try to indicate appropriate stereochemistry

Chapter 18-Slide 8

Conformations of Cyclic Monosaccharides

Draw the most stable conformation of a-D-galactopyranose

Chapter 18-Slide 9

Mutarotation

• Cyclic acetals are in equilibrium with open-chain forms
• This equilibrium allows a and b forms to interconvert
• This interconversion can result in a change in the specific rotation

Chapter 18-Slide 10

Reactions of Monosaccharides

• A. Glycoside (a type of acetal) Formation
• B. Methyl ether formation
• C. Reduction
• D. Oxidation

Chapter 18-Slide 11

Glycoside Formation

A reaction you already know!
What is the first step in the mechanism?
What structure is formed by this step?
When does the alcohol get involved?
What is the major product when alcohol is in excess?
What is the major product if water is added?

Chapter 18-Slide 12

Glycosiide Formation - II

Does the change of -OH to -NH₂ have a significant impact on the mechanism?

Chapter 18-Slide 13

Methyl Ether Formation

• Given: NaH function -
  
• What role does the dimethyl sulfate play?

Chapter 18-Slide 14

Reduction

Carbonyl reduction reactions affect the aldehyde or ketone functionality (NaBH₄, H₂/Pt...)

Chapter 18-Slide 15

Oxidation - I

Oxidation by Cu(II) or Ag(I) - a test for reducing sugars

Chapter 18-Slide 16

Oxidation - II

• Nitric Acid - oxidizes aldehydes and primary alcohols to carboxylic acids
  
• Periodic Acid - cleaves C-C bond of 1,2-diols
  

Chapter 18-Slide 17

Testing for Glucose - Important Knowledge for Control of Diabetes

• A Chemical Test:
  
• Specificity Problem: Other aldoses also react

Chapter 18-Slide 18

A More Specific Glucose Test

• An Enzymatic Test:
  
• Specificity Problem: only b-D-glycopyranose reacts

Chapter 18-Slide 19

Longer-Term Glucose Concentration Evaluation

• Glycosylation of hemoglobin
  
• Gives average measure of glucose concentration through life of red blood cells: 8-10 weeks

Chapter 18-Slide 20

Oligosaccharides - Blood Group Determinants

Chapter 18-Slide 21

Disaccharides Challenge

Given that maltose is two D-glucopyranose molecules joined by an a-1,4 glycosidic bond, draw the structure of maltose.

Chapter 18-Slide 22

Polysaccharides: Energy Storage and Structural Components

• Starch - energy storage in plants
  Amylose - up to 4000 D-glucoses linked a-1,4
  Amylopectin - shorter but highly branched polymer of D-glucoses linked both a-1,4 and a-1,6
• Glycogen - energy storage in animals
  similar to amylopectin with lower molecular weight and more branching
• Cellulose - plant skeletal component
  linear b-1,4 polymer of D-glucose

Chapter 18-Slide 23